If Sir Arthur Conan Doyle had simply written about Sherlock Holmes and all the detective’s strange mannerisms, I doubt he would have been nearly as interesting as he became when Dr. Watson entered the story.
Dr. Watson sees the world as the reader sees the world, and asks the questions that the reader would ask. Through Dr. Watson, we learn about the method behind Sherlock’s madness and are able to better form an image and opinion of his character.
In one of my (unpublished) stories, a reserved queen struggles with the growing corruption amongst her country’s noblemen. Rather than popping inside her head and narrating her thoughts, I let her explain herself naturally, as she spoke to the trusted captain of her Guard. The relationship became an important one in the story, and was infinitely better than taking the omniscient viewpoint, because it allowed the reader to learn her character as he would learn any one else’s character. You can’t know your friends by reading their thoughts, but by interacting with them.
This illustrates one of the great truths of writing: Relationship drives a story forward.
2. The side character offers a different perspective.
Different people can interpret the same event or situation completely differently. The cynic will interpret the proverbial half-empty glass as an ill omen, while the optimist will see it as a cause for cheer. In The Hobbit, the same adventure that promised excitement and a returned homeland for the Dwarves caused panic and hesitation in the Hobbit. If there’s one thing we all love, it’s a clash of personalities.
It is dull when a main character has all the answers, sees the situation clearly all the time, and survives all perils. A side character who questions the main character (for good or ill), who calls faults to account, who comes in the nick of time to save our hero—now that makes a good story. Who is Frodo without Sam? Who is Luke without Han Solo? Who is Tom without Huckleberry? We need the differing perspectives to help us have a better view of the situation as a whole.
Every side character has a different function. Let’s take, for example, Jonas from The Giver by Lois Lowry. Jonas’ friendship with the Giver shows Jonas to be respectful and conscientious; he is a learner. Oppositely, his interaction with Fiona shows his willingness to break the rules in order to show her the truth about their world; he is a teacher. His interaction with Gabriel shows his maturation from a naïve citizen to a courageous protector. Each side character reveals a new side to Jonas’ personality, and even begins to change his personality, little by little.
This happens in real life. Different people affect us differently, and different contexts require different things from us. The people at my workplace interpret my personality differently than my close friends do, but all of their perspectives create a whole, rounded perception of my true personality. Even the enemy in a fiction story tells the reader something about the main character. (As wise people have noted before: You can judge a man by his enemies.)
If you truly want your main character to live, give him a friend. Give him a competitor. Give him a spouse. Give him a student. Give him an enemy. Your character will come to life through the relationship.